拉曼光谱技术在聚合物研究中的应用进展

拉曼光谱可以提供分子的振动信息,对于聚合物分子链的构象和链间的相互作用非常敏感,能够提供聚合物固体、薄膜或溶液的物理化学特性信息,如聚合物的结构单元、空间构型、晶态结构、分子链的物理构象或分子链子链和侧基在界面间或在各向异性材料中的排列等链取向信息等。因此拉曼光谱作为一种原位无损检测技术,其衍生出的表面增强拉曼光谱技术(Surface-enhanced Raman Scattering,SERS)、变温拉曼光谱技术、共焦显微拉曼光谱技术(Confocal Raman Microscopy,CRM)、拉曼Mapping成像技术和共振拉曼散射技术(Resonance Raman scattering,RRS)等,广泛应用于物理、化学和生物医学等领域。本文从拉曼光谱的基本理论基础、拉曼光谱技术及其在聚合物研究中的最新应用进展等方面进行综述,以探索扩展拉曼光谱技术在高分子物理与化学领域中许多问题,如分子链的构象结构、分子链的结晶行为、分子链的扩散运动和共混体系相态结构变化等方面的应用。     

端基附壁高分子链的构象统计理论——Ⅲ.NRW环形链

当线形高分子链两端被固体壁吸附后,形成了“环形链”。采用半空间格点中的普通随机行走(NRW)模型讨论了环形链构象的统计理论。在NRW尾形链构象分布函数的基础上,容易推导出NRW环形链构象数的公式。当链长N→∞时,环形链构象数与自由链构象数之比随N~(-3/2)而变化。用递归方程也导出了同样的结果。环形链的均方末端距(?)与均方键长l~2之比为2N/3,与长度相同的自由链比较,有一定收缩。所导出的基本关系得到了精确计数和Monte Carlo模拟的支持。     

单分子链穿插进入双链凝聚线团的分子动力学方法模拟

用分子动力学(MD)方法模拟了聚乙烯(含300个CH₂单元)单链凝聚线团与一个双链处于平衡态的凝聚线团相互穿插的融合过程,考察分析了体系结构演变的能量信息及第三个链的引入对原来两链穿插状态的影响.研究发现,在两部分线团凝聚成一体后,各分子链一方面在表面势能各向异性的作用下试图挤入体系内部,另一方面在构象熵的驱动下又尽可能地占据体系的空间,线团的穿插是在两者共同作用下逐步完成的.第三个链的融入使得原来两个链的质心距离有所增加,链段的邻接程度减弱.     

基于三维椭球模型的构象弹性理论

基于合理的旋转异构态模型得到二维链构象分布函数,通过假设构象态消失规则得到的构象弹性理论能很好地描述高分子材料的橡胶弹性．本研究利用三维椭球模型,考虑各向异性的形变过程,对构象弹性理论进行了进一步修正;并且利用修正后的理论计算全同立构聚丙烯（iPP）的应力应变关系．理论结果成功地描述了在形变过程中,由于结构变化对自由能、熵及内能变化的影响．此外,对高分子凝聚态多尺度衔接问题也作了初步探索．     

高分子链空间构象分布的复杂性——Θ条件下能量与尺寸的分布

通过高分子链的模型的计算来研究高分子链构象的尺寸与能量的分布．结果表明，高分子的链构象在小尺寸范围布居数增多；高分子的链构象在小尺寸范围存在比较稳定的链构象状态；在小尺寸范围比较稳定的链构象多具有类似棒状等非球形结构；高分子的链构象的尺寸与能量的分布显示熵弹性具有不对称性．     

良溶剂中接枝型聚两性电解质单链构象的布朗动力学模拟

采用布朗动力学研究了在良溶剂中荷电平衡的接枝聚两性电解质(GPA)的单链构象转变行为,讨论了主链链长、支链数及电荷密度对GPA分子链构象转变的影响.研究发现,随着静电相互作用的增强,GPA分子链构象转变过程由线团、主链与支链间的折叠、链段塌缩和电荷配对形成偶极子与四极子等4个阶段构成.与线型聚两性电解质不同,GPA存在的额外支链间空间排斥与静电排斥作用随着分子结构的变化而改变,并影响构象转变行为.在强静电相互作用下,良溶剂中的GPA链由于溶剂化作用会再伸展,以保证偶极子完全配对成四极子.减小主链长度或电荷密度或增加支链数目都会增大体系的排斥力和主链的刚性,阻滞分子链的塌缩,并使得分子链再伸展的幅度增大.     

直线群及其在聚合物结构中的应用

对聚合物晶体中各种高分子链的构象进行了研究,找出了它们的三维对称性,并用直线群来表征。在此基础上,讨论了分子链在空间群中排布的规律性,即在空间群中高分子链的对称性与所在位置的对称性相呼应。并利用直线群讨论了有序-无序高分子链所在空间群转变的规律性。     

高分子链空间构象分布的复杂性——条件下能量与尺寸的分布

通过高分子链的模型的计算来研究高分子链构象的尺寸与能量的分布。结果表明，高分子的链构象在小尺寸范布居数增多；高分子的链构象在小尺寸范围存在比较稳定的链构象状态；在小尺寸范围比较稳定的链构象多具有类似棒状等非球形结构；高分子的链构象的尺寸与能量的分布显示熵弹性具有不对称性。     

聚肽分子链构象转变研究

聚肽分子链在一定的条件下可发生右旋分子链构象与左旋分子链构象间的构象转变 .文中提出了一个有关这一现象的统计理论解释 ,理论包含了两个重要的参数γ、ρ .γ与右旋和左旋分子链构象间能量差别有关 ;ρ与在左旋分子链段中启始一个右旋构象的结构单元所需能量有关 .进一步考虑了体系中酸分子存在对构象转变的影响 ,以解释新近发现的聚肽左、右旋多重分子链构象转变现象 .此外 ,运用核磁共振 (NMR)手段还对这一现象做了进一步的研究 .通过与实验结果和文献中报告的数据比较 ,证明这一理论可以很好地解释聚肽分子链中发生的左、右旋分子链构象转变     

穿插程度对高分子链结晶初始阶段的影响

用分子动力学方法模拟了具有不同穿插程度的寡链凝聚体系结晶初始阶段的有序化过程. 链间穿插程度通过径向分布函数的统计进行表征. 模拟计算结果表明, 链间的穿插程度对体系的有序化过程具有明显的影响, 穿插程度越小的体系有序化越快, 形成的有序结构也更完整. 与实验结果的比较表明, 在聚合物粒子结晶的初始阶段, 体系由穿插引起的缠结就已经起到了作用.     

Polyelectrolyte gel transitions: experimental aspects of charge inhomogeneity in the swelling and segmental attractions in the shrinking

This paper aims to provide a systematic discussion based on our experimental results both previously published and unpublished, to promote better understanding of volume-phase transitions in polyelectrolyte gels. Special attention was paid to the distribution of network charges as well as to the attractive interaction among polymer segments. From looking at how these effects appear in the swelling curves, an exploration of the nature of polyelectrolyte gel transitions was attempted. Two sorts of polyelectrolyte gels, temperature-responsive ionic gels based on N-isopropylacrylamide (NIPA) and cationic poly(ethyleneimine) (PEI) gels, were mainly employed with various modifications. The charge inhomogeneity within the gel phase was created by surfactant binding, immobilized enzyme reaction and physical entrapment of polyions. The attractive interactions holding the gel in a collapsed state were studied in comparison with phase separations of the corresponding linear polyelectrolyte. The main conclusions are summarized as follows: (i) The charge inhomogeneity exhibits a large influence on the volume transition in ionic gels. (ii) Hydrogen bonding and hydrophobic association, other than electrostatic attraction, can be considered to play an important role in the segmental association. (iii) Stably associated segments via one or more of these attractive interactions causes a large hysteresis in the swelling process, in which the repulsive interaction among the fixed charges on the network is dominant as shown in the Katchalsky's model. (iv) A distribution of "neutral but hydrophilic" moieties (e.g., ion pair or salt-linkage formed between the opposite charged groups) within the gel shows a marked effect on the temperature-induced volume collapse, the aspect of which is similar to that observed in the gels with a charge inhomogeneity.     

Cooperative conformational transitions of linear biopolymers

Conformational transitions in proteins, nucleic acids, and other biopolymers evidently play a decisive role in many biological processes, particularly in control processes. They often proceed cooperatively, i.e. the elementary process of the transition of an individual segment of these macromolecules in influenced by the state of other segments via intramolecular interactions. In general, the segments favor the same state as their neighbours. The resulting equilibrium properties of cooperative systems, e.g. the sharpness of the transitions and their dependence on the chain length, can be quantitatively explained for linear systems by the linear Ising model. The molecular causes of the cooperativity can be explained for simple model polymers.     

Critical behaviour of cooperative interactions

A simple approach to the theory of phase transitions is proposed: The cooperative interaction is operative only when the order parameter passes the critical value, which is shown to be probably constant regardless of the type of transition.     

Transitions of tethered polymer chains: a simulation study with the bond fluctuation lattice model

A polymer chain tethered to a surface may be compact or extended, adsorbed or desorbed, depending on interactions with the surface and the surrounding solvent. This leads to a rich phase diagram with a variety of transitions. To investigate these transitions we have performed Monte Carlo simulations of a bond fluctuation model with Wang-Landau and umbrella sampling algorithms in a two-dimensional state space. The simulations' density-of-states results have been evaluated for interaction parameters spanning the range from good- to poor-solvent conditions and from repulsive to strongly attractive surfaces. In this work, we describe the simulation method and present results for the overall phase behavior and for some of the transitions. For adsorption in good solvent, we compare with Metropolis Monte Carlo data for the same model and find good agreement between the results. For the collapse transition, which occurs when the solvent quality changes from good to poor, we consider two situations corresponding to three-dimensional (hard surface) and two-dimensional (very attractive surface) chain conformations, respectively. For the hard surface, we compare tethered chains with free chains and find very similar behavior for both types of chains. For the very attractive surface, we find the two-dimensional chain collapse to be a two-step transition with the same sequence of transitions that is observed for three-dimensional chains: a coil-globule transition that changes the overall chain size is followed by a local rearrangement of chain segments.     

Annealing behavior of ultrahigh molecular weight polyethylene investigated by confocal micro-Raman spectroscopy combined with two-dimensional correlation spectroscopy

Annealing was performed for ultrahigh molecular weight polyethylene (UHMWPE), including an isothermal process at 110.0°C and cooling process from 110.0 to 30.0°C. The processes were in situ investigated by confocal micro-Raman spectroscopy combined with two-dimensional correlation spectroscopy. Two phase transitions were directly observed in the annealing processes, i.e., from the amorphous phase to the intermediate phase and from the intermediate phase to the crystalline phase. The phase transitions derive from molecular chain segments sliding between different phases of UHMWPE and occur in different orders during the isothermal and cooling processes.     

Nonnucleosidic Base Surrogates: The Effect of 1,2‐Disubstituted Phenanthrenes on DNA Duplex Stability

Phenanthrene‐1,2‐dimethanol was incorporated into oligodeoxynucleotides via formation of phosphodiester bonds (cf. Scheme 1). If placed at internal positions in a DNA duplex, a strong reduction of duplex stability is observed (Table 1). Terminal attachment of stretches of phenanthrene residues, however, leads to a substantial increase in stability. The stabilization is attributed to a cooperative interaction of the phenanthrene residues of the two strands rather than to dangling end effects. Chimeric oligomers containing a stretch of six phenanthrene residues show two separate transitions (Table 2): one arising from the denaturation of the DNA stem (observable by a hyperchromic effect at 260 nm) and a second one from the denaturation of the phenanthrene part (observable by temperature‐dependant gel mobility assays). Based on these findings, a model of the chimeric hybrids is proposed, in which the phenanthrene residues stack in a zipper‐like manner on top of the DNA base pairs without disrupting the B‐form of the DNA stem (see Fig. 7).     

Local chain dynamics of a model polycarbonate near glass transition temperature: A molecular dynamics simulation

Constant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.     

Phase behavior of binary symmetric mixtures in pillared slit-like pores: a density functional approach

Density functional approach is applied to study the phase behavior of symmetric binary Lennard-Jones(12,6) mixtures in pillared slit-like pores. Our focus is in the evaluation of the first-order phase transitions in adsorbed phases and lines delimiting mixed and demixed adsorbed phases. The scenario of phase changes is sensitive to the pore width, to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters. The presence of pillars provides additional excluded volume effects, besides the confinement due to the pore walls. The effects of attraction between fluid species and pillars counteract this additional confinement. We have observed that both the increasing surface pillar density and the augmenting strength of fluid-solid interactions can qualitatively change the phase diagrams topology for the model with sufficiently strong trends for demixing. If the length of pillars is sufficiently large comparing to the pore width at low temperatures, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical points then are observed.     

Cooperative effect in ion pairing of oligolysine with heptafluorobutyric acid in reversed-phase chromatography

The retention behavior of an oligolysine mixture, consisting of two to eight residues, was examined at different concentrations of heptafluorobutyric acid (HFBA) in the mobile phase using a C18 column. A single ion record (SIR) mode of the mass spectrometer produced a distinct retention time for each oligomer component. As the concentration of HFBA increased, the retention time of each oligomer increased. Furthermore, the increase in retention time is chain-length dependent such that, the longer the oligomer chain, the more rapid was the rate that retention time increased. A closed pairing model that presumes an equilibrium between the unpaired state and the paired state with a fixed number of HFBA molecules was used to analyze the retention factor as a function of [HFBA]. Curve fitting gave estimates of the ion-pairing equilibrium constant (K(ip,m)), the distribution constant of paired oligolysine (K(D,ip)), and the number of paired HFBA for each oligolysine (n). The plot of the fraction of paired oligolysine in the mobile phase, estimated from K(ip,m) and n as a function of [HFBA], revealed a cooperative effect. In contrast, an open pairing model that assumes independent pairing of HFBA with each residue failed to describe the observed retention behavior.     

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.